To provide a building foundation structure which dispenses with conventional back-fill of soil by heavy equipment when a foundation of a house, an apartment house, a shop, a factory, a warehouse, or the other building is constructed, and has a downsized foundation, and to provide a building foundation construction method.

According to the building foundation structure, a mat foundation 10 integrally formed of a foundation bottom slab 10a and raised portions 10b is constructed, then styrofoam blocks 11 are laid on an upper surface of the foundation bottom slab 10a, and reinforcements of a concrete slab 12 are arranged on upper surfaces of the laid styrofoam blocks 11, followed by placing concrete. By virtue of this structure, the lightweight styrofoam blocks 11 are laid on the upper surface of the foundation bottom slab 10a, and therefore load applied to the foundation bottom slab 10a is reduced, which leads to weight reduction of the entire foundation structure, to thereby achieve downsizing of the foundation bottom slab 10a even on weak ground.